This invention has to do with locks, and relates especially to locks that require two keys for their operation, as is typically true, for example, in locks for safe deposit boxes and similar applications.
Locks of that general type ordinarily require a correspondingly large number of parts, making them relatively expensive to manufacture and service. In particular, if each key drives a distinct and independent set of tumblers the number of moving parts in the lock may approach twice that required in a single-key lock.
Many presently available dual-key locks employ two sets of pivoted tumblers each of which is independently operated by one of the keys. In such locks the radial length of the tumblers is severely limited by the conventional size of the lock housing. That shorter tumbler length increases the angle through which the tumbler must swing for any given bit height of the key, increasing correspondingly the range of angles at which the key engages the various tumblers in bolt-releasing position. That increased range of working angles between key and tumbler tends to reduce accuracy, limiting the number of different key configurations that can be provided with a given number of tumblers.
It has also been proposed to construct a dual key lock with a single stack of floating tumblers which are engaged by the two keys at spaced positions along their length. That concept is disclosed, with a variety of auxiliary features, in a series of patents by Roy T. Ellis, of which U.S. Pat. No. 3,127,759 is illustrative. However, the tumblers of Ellis' single stack are of complex shape and require numerous auxiliary levers and secondary tumblers for their operation. Moreover, the gate positions on each tumbler's primary working edge must take account of the bit height of both the keys. Thus, if the key combination is to be changed by replacing one set of tumblers by a set corresponding to a new pair of keys, a very large number of different tumbler forms must be kept on hand. If key A has six bit heights, for example, and key B has eight, the possible key combinations may involve 48 different positions of the primary gate, all of which must be available if all possible combinations are to be provided. In contrast, the more conventional locks with two independent stacks of tumblers involve only six gate configurations for one tumbler and eight for the other, or a total of 14 species. The increased complexity and expense of the Ellis lock in changing keys is evident.